Project Summary: Nutrition and dietary adaptation shape all aspects of animal physiology across taxa, including the composition and maintenance of intestinal microbiota, which, in turn, influence host animal metabolic responses. The reciprocal interactions between diet, host signaling networks, and microbiota define a physiological rheostat that governs host metabolism. Importantly, when these interactions are misregulated, the result is often metabolic dysfunction and disease. Thus, there is a critical need to explore the distinct cellular and molecular host signaling mechanisms that promote diet-microbe interactions and influence host physiology. The overall goal in this proposal is to investigate diet-dependent host signaling mechanisms, driven by the evolutionarily conserved, innate immune signaling pathway transcription factor NFkB, that influence intestinal microbiota homeostasis. I provide evidence that NFkB transcription factor function in the Drosophila intestine can govern microbiota maintenance and metabolic signaling pathway activity in response to specific changes in dietary macronutrients, putatively influencing microbiota-regulated aspects of host health and dietary adaptation. More specifically, I find that, in response to high carbohydrate / low protein dietary macronutrient ratios, NFkB activity can modulate the function of the 4EBP/TOR signaling pathway, a conserved regulator of physiology that couples nutrition, cellular energy homeostasis and mRNA translation. I hypothesize that intestinal NFkB function is required to maintain microbiota homeostasis in response to dietary changes through influencing host 4EBP/TOR signaling pathways. To address this hypothesis, I aim to utilize the fruit fly model, Drosophila melanogaster, to; (i) assess NFkB/4EBP/TOR integration by characterizing the cellular interactions of these signaling pathways and their impact on microbiota maintenance during dietary adaptation. A result of disruption of these signaling interactions is a significant change in mRNA translation within the intestine, thus I also plan to; (ii) explore the influence of intestinal translational on microbiota maintenance. Lastly, I anticipate that the changes I see in microbiota composition during dietary adaptation are impacting host physiology. Specifically, based upon current literature, I aim to; (iii) investigate the effect of NFkB-and- diet-dependent changes in microbiota maintenance on host proteolytic activity and amino acid homeostasis. Exploiting Drosophila to explore evolutionarily conserved host signaling mechanisms involved in maintaining commensal microbes under changing dietary conditions holds promise for developing integrative diet- microbiota interaction frameworks that are useful for optimizing human health.